Hibernate for Beginners

Hibernate Kick off

1. Hibernate Introduction2. Why to use Hibernate3. Steps to use/configure hibernate in an application4. simple case study 5. Transaction in Hibernate6. Using Criteria in Hibernate 7. Caching in Hibernate

Hibernate Introduction:

Hibernate is a powerful, high performance object/relational persistence and query service. Hibernate lets you develop persistent classes following object-oriented idiom - including association, inheritance, polymorphism, composition, and collections. Hibernate allows you to express queries in its own portable SQL extension (HQL), as well as in native SQL, or with an object-oriented Criteria and Example API. Object/Relational Mapping Hibernate Dual-Layer Cache Architecture Highly scalable architecture J2EE integration Object Relational tool ( JDO, Hibernate, and iBatis SQL Maps, TopLink) Hibernate supports these following databases:

Why to use Hibernate

In traditional approach:

o Too many SQL statements

o Manually handled associations

o Database dependent

Hibernate architecture

Steps to use/configure hibernate in an application:

A. Installing the Hibernate core and support JAR libraries into your project

B. Creating a Hibernate.cfg.xml file to describe how to access your database

C. Selecting appropriate SQL Dialect for the database.

D. Creating individual mapping descriptor files for each persistable Java classes

A. Install the Hibernate core and JAR libraries

Steps to use Hibernate

B. Creating a Hibernate.cfg.xml :

Before Hibernate can retrieve and persist objects for us, we need to tell it the settings about our application. For example,

which kind of objects are persistent objects? Which kind of database are we using? How to connect to the database? What is the size of Connection pool?

There are three ways to configure Hibernate in total:

1. XML configuration

2. programmatic configuration

3. properties file configuration.


XML Configuration

9. Steps to use Hibernate

C. Selecting appropriate SQL Dialect for the database. The dialect property determines the dialect to be used when generating queries. Hibernate supports dialects for all popular Relational Database Management Systems (RDBMS), such as DB2 or Oracle™. Therefore, for example, if you use Oracle during development and want to move to DB2 in production, changes will only be required in the hibernate.cfg.xml file.


A. Creating individual mapping/descriptor files for each persistable Java classes/Table in DB

JAVA XML RDBMS POJO Class HBM File A Table


A Table in Database

12. Steps to use Hibernate

CREATE TABLE BOOK (

ISBN VARCHAR(50) NOT NULL,

NAME VARCHAR(100) NOT NULL,

PRICE INT NOT NULL,

PRIMARY KEY (ISBN) );

Java (POJO):


Hibernate-mapping: (*.hbm.xml)


0

4 simple case study

simple case study

Creating global session factory:For an application using Hibernate as O/R mapping framework, a global session factory should be created and accessed through a particular interface. Here we use a static variable for storing the session factory. It is initialized in a static block when this class is loaded for the first time.

Using the SessionFactory in Application: SessionFactory factory = HibernateUtil.getSessionFactory(); Session session = factory.openSession();

try { //…………………..// Using the session to retrieve objects

//………………………………. } finally {

session.close(); }

SessionFactorys are immutable. The behaviour of a SessionFactory is controlled by properties supplied at configuration time.

Simple Case Study

Retrieving objects

Book book = (Book) session.load(Book.class, isbn);orBook book = (Book) session.get(Book.class, isbn);

What’s the difference between load() and get() ?

The first difference is that when the given ID could not be found, load() will throw an exception “org.hibernate.ObjectNotFoundException”, while get() will return a null object.

The second difference is that load() just returns a proxy by default and database won’t be hit until the proxy is first invoked. The get() will hit the database immediately.

Simple Case Study

Using HQL (Hibernate Query Language):

If you are sure that there will be only one object matching, you can use the uniqueResult() method to retrieve the unique result object.

So, the code would appear as:

19. Simple Case Study

persisting Objects:

For saving a newly created object, we can use the save() method. Hibernate will issue an INSERT statement. session.save(book);

For updating an existing object, we can use the update() method. Hibernate will issue an UPDATE statement. session.update(book);

For deleting an existing object, we can use the delete() method. Hibernate will issue a DELETE statement. session.delete(book);


Id Generation in Hibernate

ID generation in Hibernate There are three approaches to set ID:

a. Sequence

b. Identity

c. Native

Simple Case Study

ID generation in Hibernate There are three approaches to set ID: a.Sequence

To generate an ID is to use an auto-incremented sequence number. For some kinds of databases (including HSQLDB), we can use a sequence/generator to generate this sequence number:


b. Identity:

To generate an auto-incremented sequence number is to use an identity column of a table.

Simple Case Study

c. Native:

most suitable strategy to use for your database


primary key generation using multiple columns:

Table:


Java (POJO Class)


Mapping (an HBM file)


5. Transaction in Hibernate:

Transaction in Hibernate:

Example: select

31. Transaction in Hibernate

Example: update


Example: delete

Transaction in Hibernate

6 Using Criteria in Hibernate

Sometimes we need to build up a query dynamically in our application, e.g. for an advanced search function.“Criteria Queries” is an alternative way of HQL query.The traditional method of doing this is to generate a HQL statement, or SQL statement if not using Hibernate, by string concatenation. The problem for this method is making your code hard to maintain because of the hard reading statement fragments.

Traditinal approach:

if (startDate != null) { if (firstClause) {

query = query + " where ";} else {

query = query + " and "; query += " s.date >= '" + startDate + "'";}// And so on...


Hibernate Criteria:

Criteria criteria = session.createCriteria(Book.class);

if (startDate != null) {criteria.add(Expression.ge("date",startDate);

}

if (endDate != null) {criteria.add(Expression.le("date",endDate);

}

List results = criteria.list();

Transaction in Hibernate

SELECT * FROM ORDERS WHERE ORDER_ID=’1092’;

In Cretira it would become: List orders= session.createCriteria(Order.class) .add(Restrictions.eq(“orderId”,”1092”)) .list();

SELECT O.*, P.* FROM ORDERS O, PRODUCT P WHERE O.ORDER_ID=P.ORDER_ID AND P.ID=’1111’;Would become

List orders = session.createCriteria(Order.class) .setFetchMode(“products”,FetchMode.JOIN) .add(Restrictions.eq(“id”,”1111”)) .list();


This criteria query corresponds to the following HQL query:

from Book bookwhere book.name = 'Hibernate for Beginners'



•Restriction.between is used to apply a "between" constraint to the field.

•Restriction.eq is used to apply an "equal" constraint to the field.

•Restriction.ge is used to apply a "greater than or equal" constraint to the field.

•Restriction.gt is used to apply a "greater than" constraint to the field.

•Restriction.idEq is used to apply an "equal" constraint to the identifier property.

•Restriction.in is used to apply an "in" constraint to the field.

•Restriction.isNotNull is used to apply an "is not null" constraint to the field.

•Restriction.isNull is used to apply an "is null" constraint to the field.

•Restriction.ne is used to apply a "not equal" constraint to the field.

IN:

criteria.add(Restrictions.in("newCourseID", courseIDs));

7. Caching in Hibernate:

Hibernate supports the caching of persistent objects at different levels:

1st level of caching

2nd Level of caching

41. Caching in Hibernate

1. 1st level of caching: Suppose we get an object with same

identifier for two times within a session, will Hibernate query the database for two times?


If we inspect the SQL statements executed by Hibernate, we will find that only one database query is made. That means Hibernate is caching our objects in the same session. This kind of caching is called “first level caching”, whose caching scope is a session.

But how about getting an object with same identifier for two times in two different sessions?


2nd Level of caching:


We will find that two database queries are made. That means Hibernate is not caching the persistent objects across different sessions by default.

We need to turn on this “second level caching” whose caching scope is a session factory.


To enable 2nd level of caching, update the ‘hibernate.cfg.xml’

To monitor the caching activities of Hibernate at runtime, we can add the following line to the log4j configuration file “log4j.properties”. log4j.logger.org.hibernate.cache=debug

46 Caching in Hibernate

Thank you.

Hibernate for Beginners

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